System and method for desalination of water using a graphite foam material

ABSTRACT

A condenser or heat exchanger includes a circulation system for moving a cooling fluid, and a graphite foam in thermal communication with the circulation system. The condenser or heat exchanger can be used to remove water, or more particularly freshwater from water vapor or steam produced from seawater.

RELATED APPLICATION

This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/568,605, filed on Dec. 8, 2011, which is incorporated by reference herein.

TECHNICAL FIELD

Various embodiments described herein relate to a system and method for desalination of water using a graphite foam material. The system and method includes the use of heat to produce steam from seawater and condensation of the steam to produce a source of freshwater for human consumption.

BACKGROUND

An increase in worldwide population has led to the increase in demand for freshwater for human consumption and irrigation. Over 99% of the world's freshwater comes from tapping a diminishing source of the world's rivers, lakes, and groundwater locations that are becoming less dependable as some are reaching maximum capacities. With only 1% of the world's water supply available for human use in a constantly expanding worldwide population, clean water is becoming the most important commodity in water-stressed regions. The increase in demand for freshwater has been most evident in dry areas where rainwater is scarce and groundwater sources are drying up such as: the Middle East, Australia, and the American West and Southwest, to name a few.

Clean water is necessary for irrigation in arid regions where occupants rely on importing most of their food because agriculture is too expensive or not possible. Although clean water is basic utility in water-rich and developed regions, the arid and less developed regions of the world do not have access to clean water.

Most of the earth's surface, about 71%, is covered with water. However, most of the water is in saltwater oceans. Of course, salt water is unfit for human consumption. Water can be desalinated. The two most common options for water production include non-thermal/pressure/membrane processes, and thermal processes. The non-thermal/pressure/membrane processes include reverse osmosis (RO), filtration, sludge, and the like. The thermal processes include multi-stage flash (MSF) evaporation, multi-effect distillation (MED), and low-temp thermal desalination (LTTD). Generally, water treatment and desalination methods require capital intensive equipment and facilities that become more expensive in regions that are arid and underdeveloped. In addition, the thermal processes require energy which is also expensive. Large amounts of energy are necessary for both operation and sustainment of a multi-stage flash evaporation plant or a multi-effect distillation plant. The energy and cost requirements many times eliminate these types of plants as desalination solutions unless there is a source of waste heat near the site. Increasing the efficiency would bring down the cost of operation. In addition, reducing the capital expenditures would also add the MSF and MED solutions to other desalination solutions. Adding additional desalination solutions reduces the chances that people resort to drinking water from polluted sources. Consumption of polluted water affects the health of approximately 1.2 billion people and contributes to 5 million deaths each year from water-related diseases such as cholera, schistosomiasis, and malaria.

The MSF and MED plants generally have a condenser which is used to condense or distill water from the water vapor in the MSF or MED device. Condensers used for MSF and MED plants generally employ shell and tube technology. Expensive materials, such as copper, nickel, aluminum brass, stainless steel and titanium, are also used in these plants. These materials drive up the capital expenditure associated with an MSF or MED plant and also may price such a plant so that it is no longer a choice for a desalination solution.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a multi-stage flash (MSF) thermal process used to desalinate seawater, according to an example embodiment.

FIG. 2 is a schematic view of a stage of the multi-stage flash (MSF) thermal process plant shown in FIG. 1, according to an example embodiment.

FIG. 3 is a schematic view of a multi-effect distillation (MED) thermal process used to desalinate seawater, according to an example embodiment

FIG. 4 is a perspective view of a condenser that could be used either in the MSF plant or the MED plant, according to an example embodiment.

FIG. 5 is a cross section view of a portion of the condenser 500 shown in FIG. 4, according to an example embodiment.

FIG. 6 shows another view of the condenser along with several subassembly portions which form the graphite foam heat exchanger (GFHX) made with graphite foam, according to an example embodiment.

FIG. 7 shows a Vertical Tube Evaporator (VTE) with steam supplied on the shell side, according to an example embodiment.

FIG. 8. shows an Ocean Thermal Energy Conversion (OTEC) system having an Open Claude cyle, according to an example embodiment.

FIG. 9 shows a low-temperature thermal desalination system which is part of an Ocean Thermal Energy Conversion (OTEC) system having an Open Claude cyle which includes a flash chamber, according to an example embodiment.

FIG. 10 shows a system for desalination of seawater which includes a Closed Cycle (Rankine) used for power production, and the Open Cycle used for water production, according to an example embodiment.

DETAILED DESCRIPTION

FIG. 1 is a schematic view of a multi-stage flash (MSF) thermal process plant 100 used to desalinate seawater, according to an example embodiment. The MSF plant 100 includes a series of stages 110, 112, 114. The stages 110, 112, 114 include a heat exchanger 310, 312, 314. One or more of the heat exchangers 310, 312, 314 include a graphite foam heat exchange surface. The stages 110, 112, 114 also have condensation collectors 130, 132, 134 that receive condensation from the heat exchangers 310, 312, 314. The MSF plant 100 has a cold end 120 and a hot end 122. The stages 110, 112, 114 are intermediate to the cold end 120 and the hot end 122. The stage 110 has the lowest temperature and is closest the cold end 120. The stage 134 is the warmest of the stages and is positioned closest to the hot end of the MSF plant 100. The stages 110, 112, 114 have different pressures corresponding to the boiling points of water at the stage 110, 112, 114 temperatures. The MSF plant 100 includes a brine heater 124 at the hot end 122 of the MSF plant 100. As the seawater passes through the heat exchangers 310, 312, 314 and is then placed into the brine heater 124 where it is heated. It should be noted that the seawater or brine entering the brine heater 124 has substantially the same concentration of salt as the seawater entering the cold end 120 in the first stage 110. In the brine heater 124, the seawater or brine is heated to a temperature where water vapor will flash or boil off after the seawater enters the stage 114. The concentration of salt in the seawater increases in the stage 114 where some freshwater flashes off. Brine is actually water with a higher concentration salt than the incoming seawater. The brine in stage 114 is then pumped into the next stage 112 where another portion of freshwater is flashed or boiled off. The brine in stage 112 is then pumped into the stage 110 where the temperature and pressure allows another portion of water to flash or boil off. In each stage, freshwater condenses on the heat exchanger 314, 312, 310 and drops onto the condensate collector 134, 132, 130. Condensate from the condensate collectors 134, 132, 130 is then collected and the freshwater is output from the MSF plant 100. The condensate, or freshwater, may have to undergo further water treatment before it is potable. The condensate could also be used for irrigation.

In operation, cool or cold seawater is pumped through the stages 110, 112, 114. Cold seawater is pumped through the heat exchanges 310, 312, 314 which transfers heat to the incoming water en route to the brine heater 124. The cold water is heated while some of the flashed or evaporated water is condensed out on the heat exchanger surfaces of the heat exchangers 310, 312, 314. When the MSF plant 100 is operating in steady state, feed water or the seawater at the cold inlet temperature flows, or is pumped, through the heat exchangers 310, 312, 314 in the stages 110, 112, 114 and warms up during each stage. When the seawater reaches the brine heater 124 at the hot end, it already has a raised temperature making the heat transfer more efficient. At the brine heater 124, an amount of additional heat is added. After the brine heater 124, the seawater/brine mixture flows through valves back into the stages 114, 112, 110 which each have progressively higher salinity, lower pressure and temperature. As the seawater or brine flows back through the stages the fluid mixture is now called brine, to distinguish it from the inlet seawater. In each stage, as the brine enters, its temperature is above the boiling point at the pressure of the stage, and a small fraction of the brine water boils (“flashes”) to steam thereby reducing the temperature until a liquid-vapor equilibrium state is reached. The resulting steam is warmer than the feed water in the heat exchanger. The steam cools and condenses against the heat exchanger 310, 312, 314 heat transfer surfaces, thereby heating the feed water as described earlier.

The total evaporation in all the stages 110, 112, 114 can be up to approximately 15% of the seawater flowing through the system, depending on the range of temperatures used. It should be noted that three stages are shown and there may be more or less stages for a particular MSF plant. With increasing temperature in the stages, there are growing difficulties of scale formation and corrosion. A maximum appears to be 120° C., although scale avoidance may require temperatures below 70° C.

The feed water carries away the latent heat of the condensed steam, maintaining the low temperature of the stage. The pressure in the chamber remains constant as equal amounts of steam are formed when new warm brine enters the stage and steam is removed as it condenses on the tubes of the heat exchanger. The equilibrium is steady state, because if at some point more vapor forms, the pressure increases and that reduces evaporation and increases condensation.

In the final stage the brine and the condensate have a temperature near the inlet temperature. Then the brine and condensate are pumped out from the low pressure in the stage 110 to the ambient pressure. The brine and condensate still carry a small amount of heat that is lost from the system when they are discharged. The heat that was added in the heater makes up for this loss.

The heat added in the brine heater 124 usually comes in the form of hot steam from an industrial process co-located with the desalination plant. The steam is allowed to condense against tubes carrying the brine on the shell-side of a conventional shell and tube heat exchanger (similar to the stages).

MSF distillation plants, especially large ones, are often paired with power plants in a cogeneration configuration. Waste heat from the power plant is used to heat the seawater, providing cooling for the power plant at the same time. This reduces the energy needed by one-half to two-thirds, which drastically alters the economics of the plant, since energy is by far the largest operating cost of MSF plants.

FIG. 2 is a schematic view of a stage 110 of the MSF thermal process plant 100 shown in FIG. 1, according to an example embodiment. The stage 110 of the MSF plant 100 is essentially one compartment 201 in a series of compartments that form the MSF plant. The stage 110 includes heated liquid brine 210 that is at a temperature where at least a portion of the heated liquid brine could evaporate or turn to water vapor. In other words, the heated brine 210 is capable of flashing over to steam. The stage 110 also includes a demister 220 and a distillation collection device 130. The demister 220 re-entrains brine. Brine droplets form on the demister 220 and drop back into the heated brine 210. The seawater travels through tubes, 311, 312, 313, 314 and 315, in a heat exchanger (310). The heat transferring surface is attached to the tubes 311, 312, 313, 314 and 315. The heat transferring surface includes graphite foam. Graphite foam can be attached directly to the tube with a metallic, thermally conductive epoxy, such as AREMCO 568 available from AREMCO Products of Valley Cottage, N.Y., USA. In another embodiment, the graphite foam can be attached to fins which are in turn attached to the tubes 311, 312, 313, 314 and 315 carrying the seawater. In one embodiment, the heat exchanger 310 can be made from a tubesheet and set of tubes made of aluminum. The tubes can be welded to the tubesheet using friction stir welding (FSW) which allows aluminum to be used with lessened corrosion problems. The graphite foam is depicted by reference numbers 321, 322, 323, 324 and 325.

The graphite foam material is a material having highly ordered graphitic ligaments and is as thermally conductive as bulk aluminum, at 20% the weight. Graphite foam is dimensionally stable and has a low coefficient of thermal expansion (˜2-4 in/° C.). Graphite foam also is open porous, absorbs sound, and reflects or scatters RF/EMI/EMP. Thus, graphite foam is a lightweight thermal management material that enables designers to manage multiple aspects of a design problem with one material. It is believed that this material has many applications and will lead to radically new concepts in thermal, acoustic, RF/EMI signature management. Potential applications for the graphite foam material include power electronics cooling where a ten-fold increase in cooling potential over traditional heat sinks has been demonstrated. Other uses include transpiration/evaporative cooling for electronics and leading edges. The graphite material can also be used in radiators for all types of vehicles, such as heavy vehicles, racing vehicles, aircraft, fuel cell vehicles, and space vehicles. The material can be used to shield EMI (electromagnetic interference), and for thermal and acoustic signature management. Still another application is for batteries and battery cooling.

FIG. 3 is a schematic view of a multi-effect distillation (MED) thermal process plant 400 used to desalinate seawater, according to an example embodiment. The MED evaporator 400 includes a plurality of consecutive cells (or effects) 410, 412, 414 maintained at decreasing levels of pressure (and temperature) from the first (hot) cell 410 to the last one (cold) 414. Each cell 410, 412, 414 includes a horizontal tube bundle associated with a heat exchanger 430, 432, 434. Each cell or effect 410, 412, 414 includes a sprayer 440, 442, 444. The top of the bundle or heat exchanger 430, 432, 434 is sprayed with the seawater make-up from the respective sprayer 440, 442, 444. The seawater sprayed onto the heat exchanger 430, 432, 434 then flows down from tube to tube by gravity. Although the horizontal tube evaporator MED configuration is the most widely used, a Vertical Tube Evaporator (VTE) has been demonstrated as a viable configuration for MED

FIG. 7 shows a Vertical Tube Evaporator (VTE) 700 with steam supplied on the shell side 702, according to an example embodiment. The VTE 700 is a form of MED that utilizes tubes 710 arranged vertically where film evaporation occurs inside the tubes 710 and steam is supplied to the outsides 720 of the tubes 710. Horizontal MED is the more common orientation because it employs more efficient shell side flow distribution than VTE. Application of graphite foam 730 on the outsides of the tubes will greatly increase the surface area and heat transfer coefficient of the evaporator, allowing the size of the evaporator to be reduced. For the sake of clarity, the graphite foam 730 is shown only partially covering the tubes 720. The remaining tubes 710 are shown uncovered. It should be noted that in the completed embodiment, the graphite foam 730 covers the exterior surfaces of the tubes 710. A reduction in the size of the evaporator will allow for better shell side flow distribution because the volume in which the steam condenses has been reduced; however, the application of graphite foam 730 allows a smaller unit to maintain the same or better thermal duty. The application of graphite foam 730 in the VTE configuration has advantages over the same application in the MED and MSF processes because the graphite foam 730 is only coming in contact with the steam supply which does not carry any salt or other potentially corroding compounds, as shown in FIG. 7

Turning back to FIG. 3, heating steam is introduced inside the tubes of the heat exchanger 430, 432, 434. Since tubes are cooled externally by make-up flow sprayed onto the heat exchangers 430, 432, 434, steam condenses into distillate (freshwater) inside the tubes. At the same time sea water within the cells or effects 410, 412, 414 warms up and partly evaporates by recovering the condensation heat (latent heat). Due to evaporation, remaining sea water slightly concentrates when flowing down the bundle and gives brine (water with a higher concentration of salt than incoming seawater) at the bottom of the cell or effect 410, 412, 414. For example, some water flashes in cell 410 and the output 450 of cell 410 will be brine having a slightly higher concentration than incoming seawater. In cell 412, additional water flashes and is removed from the saltwater resulting in still more concentrated brine at the output 452. The same thing happens in the last cell 414 with the output 454 being labeled brine. The vapor raised by seawater evaporation is at a lower temperature than heating steam. However it can still be used as heating media for the next effect where the process is repeated. The decreasing pressure from one cell to the next one allows brine and distillate to be drawn to the next cell where they will flash and release additional amounts of vapor at the lower pressure and temperature. This additional vapor will condense into distillate inside the next cell.

This process is repeated in a series of cells or effects 410, 412, 414 (thus the name Multiple Effect Distillation). FIG. 3 shows an MED (see sketch with 3 effects). In the last cell 414, the produced steam condenses on a shell and tube heat exchanger 436. This exchanger, called a “distillate condenser” is cooled by seawater which is pumped through the tubes of the distillate condenser 436. The distillate condenser 436 includes graphite foam on the shell side to more effectively transfer heat between the cool seawater and the steam or vapor side of the distillate condenser 436. At the outlet of the distillate condenser 444, part of the warmed seawater is used to make-up for evaporated water; the other part is rejected to the sea. Brine and distillate are collected from cell to cell until the last one from where they are extracted by centrifugal pumps. The thermal efficiency of such an MED evaporator can be quantified as the number of kilograms of distillate produced per one kilogram of seawater introduced in the system. Such number is called the Gain Output Ratio (GOR) and is a measure of system efficiency. It should be noted that in this embodiment of the heat exchanger, the graphite foam is used in the evaporator section as well because the enhanced surface area promotes pool boiling and the compact porous structure reduces the residence time of bubbles which reduces fouling on thermal surfaces

FIG. 3 also includes a first condenser 436 shell and tube configuration and a second condenser 936 shell and plate configuration. In both configurations 436 and 936, graphite foam is used as part of the heat transfer surface in the condenser or heat exchanger used to condense water from steam produced in either the MFD device 100 or the MED device 400.

FIG. 4 is a perspective view of a condenser 500 that could be used either the MSF plant 100 or the MED plant 400, according to an example embodiment. FIG. 5 is a cross section view of a portion of the condenser 500 shown in FIG. 4, according to an example embodiment. Now referring to both FIGS. 4 and 5, the condenser 500 will be discussed in more detail.

The condenser 500 is a highly efficient Graphite Foam Heat Exchanger (GFHX) using a hybrid heat exchanger (HX) in a shell & plate-fin configuration. The condenser 500 includes a first tubesheet 550 and a second tubesheet 552. The first tubesheet 550 includes openings for various tubes that will be attached to the openings.

Similarly, the second tubesheet 552 includes openings for various tubes that will be attached to the openings. The first tubesheet 550 corresponds to one end of a tube and the second tubesheet 552 corresponds to the other end of the tube attached between the first tubesheet 550 and the second tubesheet 552. Graphite foam surrounds the tubes between the two plates. The graphite foam is in thermal communication with the tubes as shown in FIG. 5. FIG. 5 shows three tubes 510, 512, 514 through which seawater is passed. As shown in FIG. 5, the tubes 510, 512, 514 are made of aluminum, in one example embodiment. In most embodiments, the material used is corrosion resistant so that the structure will last for a long time. Seawater is very corrosive when it contacts steel material. The graphite foam material 520, 522 is sandwiched between the tubes 510, 512, 514 and isolated from the seawater The graphite foam material enhances the thermal transfer efficiency between the tubes and fin material. It should be noted that in a design there are generally many more tubes than the three shown in FIG. 5. Note, there are numerous openings in the first tubesheet 550 of FIG. 4 and each one will generally have a corresponding tube or multi-hollow tube extrusion (MHE). The graphite foam material generally has more surface area than the metal fins and is more thermally conductive, so the heat transfer capability of the resulting structure is enhanced when compared to a condenser that has only metal fins. In another embodiment, graphite foam fins are attached to the tubes 510, 512, 514 and the graphite foam is bonded to the fins to provide increased surface area and increased heat transfer for the tubes.

A structure 570 formed by the first tubesheet 550 and the second tubesheet 552, the tubes and the graphite material is placed in a shell 530. The shell 530 has a seawater inlet 532 at one end and a seawater outlet 542 at the other end. The shell 530 also has a steam inlet 534 and a condensate outlet 544 Water that is condensed on the graphite foam passes out outlet opening 544 of the shell 530. It should be noted that the shell 530 can be made of any material. In low pressure systems, the shell does not have to be a pressure vessel and can be made out of less expensive materials, such as fiberglass. Of course, the structure 570 must include a substructure to fit tightly to the shell 530, in one embodiment, so as to prevent a bypass condition where the incoming steam does not pass over the tubes, such as tubes 510, 512, 514, in the structure 570.

FIG. 6 shows another view of the condenser 636 along with several subassembly portions which form the graphite foam heat exchanger (GFHX) made with graphite foam, according to an example embodiment. The condenser 636 has a Hybrid, Shell & Plate or Plate 610 configuration. The GFHX 636 includes low-cost, marine grade Aluminum alloy extrusions with the foam bonded to the multi-hollow tubes 604. This creates a hybrid (Shell & Plate) GFHX that is very efficient and inexpensive to build. The low-pressure shell (not shown but similar to that shown in FIG. 4) enables the use of inexpensive composites and fiberglass materials as the shell materials. A thermally conductive epoxy bonds the aluminum 602, the graphite foam 610 and Multi-hollow extruded (MHE) tube 604. Joining by use of brazing techniques can also be used in some embodiments. The corrosion points that stem from brazing in such a device can be avoided by using a suitable coating. Bonding allows the use of marine grade aluminum alloys, such as 5052, 5086, 6061 or 6063 aluminum alloys to be used and allows the material strength of these metals to be maintained. A hybrid, shell and plate-fin or enhanced tube construction is a relatively simple to manufacture technique. Furthermore, the use of Friction Stir Welding (FSW) on tube sheet ends can save construction cost and reduce corrosion, and use of graphite foam enhances heat transfer and resulting water (condensate production). The enhanced heat transfer and resulting water production may result in reduced size of condensers. In addition, the cost in dollars per unit of water produced is also reduced when considered as a desalination system.

This has been a detailed description of some exemplary embodiments of the invention(s) contained within the disclosed subject matter. Such invention(s) may be referred to, individually and/or collectively, herein by the term “invention” merely for convenience and without intending to limit the scope of this application to any single invention or inventive concept if more than one is in fact disclosed. The detailed description refers to the accompanying drawings that form a part hereof and which shows by way of illustration, but not of limitation, some specific embodiments of the invention, including a preferred embodiment. These embodiments are described in sufficient detail to enable those of ordinary skill in the art to understand and implement the inventive subject matter. Other embodiments may be utilized and changes may be made without departing from the scope of the inventive subject matter. Thus, although specific embodiments have been illustrated and described herein, any arrangement calculated to achieve the same purpose may be substituted for the specific embodiments shown. This disclosure is intended to cover any and all adaptations or variations of various embodiments. Combinations of the above embodiments, and other embodiments not specifically described herein, will be apparent to those of skill in the art upon reviewing the above description. 

What is claimed:
 1. A vapor-fin heat exchanger comprising: aluminum multi-hollow extruded (MHE) tubes; and graphite foam thermally coupled to an exterior surface of the tubes.
 2. The vapor-fin heat exchanger of claim 1 wherein a cooling fluid is moved through the tubes and water vapor condenses on the graphite foam.
 3. The vapor-fin heat exchanger of claim 2 in use of thermal and non-thermal condensation of water for irrigation or drinking.
 4. The vapor-fin heat exchanger of claim 1 used for flash evaporation in a multi-stage flash evaporation system.
 5. The vapor-fin heat exchanger of claim 4 used to enhance distallation in a multi-effect distillation evaporation system.
 6. The vapor-fin heat exchanger of claim 1 housed within a fiberglass shell.
 7. The vapor-fin heat exchanger of claim 1 wherein the graphite foam has machined channels therein for increasing the surface area of the graphite foam.
 8. The vapor-fin heat exchanger of claim 1 wherein the graphite foam has machined channels therein for fluid management.
 9. The vapor-fin heat exchanger of claim 1 wherein the graphite foam is bonded to substantially round condenser tubes with thermally conductive adhesive, soldering, or brazing.
 10. The vapor-fin heat exchanger of claim 1 wherein the graphite foam is bonded to substantially flat condenser tubes with thermally conductive adhesive, soldering, or brazing.
 11. The vapor-fin heat exchanger of claim 1 wherein the graphite foam is used with a metallic foams to enhance heat transfer.
 12. A vapor-fin heat exchanger comprising: an aluminum multi-hollow extruded (MHE) set of tubes; and graphite foam bonded on an exterior surface of the set of tubes; and a cooling fluid from a refrigeration cycle being moved through the set of tubes so that water condenses on the graphite foam and exterior surface to produce water for irrigation or drinking.
 13. The vapor-fin heat exchanger of claim 12 within a flash evaporation system.
 14. The vapor-fin heat exchanger of claim 12 within a distillation evaporation system.
 15. The vapor-fin heat exchanger of claim 12 wherein, the vapor-fin heat exchanger is substantially horizontally in operation.
 16. The vapor-fin heat exchanger of claim 12 wherein the vapor-fin heat exchanger is substantially vertically in operation.
 17. The vapor-fin heat exchanger of claim 12 further comprising a polymer, the graphite foam including a polymer coating on the heat transfer surfaces.
 18. The vapor-fin heat exchanger of claim 12 further comprising a polymer, the graphite foam can be bonded to heat transfer surfaces using the polymers and non-polymeric coatings.
 19. The vapor-fin heat exchanger of claim 12 further comprising a plastic tube for carrying a cooling fluid, the graphite foam can be bonded to plastic tube using using polymeric, non-polymeric, or thermally conductive epoxy.
 20. The vapor-fin heat exchanger of claim 12 further comprising a Diamond Like Carbon (DLC) coating on the graphite foam. A Closed Cycle Ocean Thermal Energy Conversion system which expends cold sea water, the system further comprising a flash evaporization system to produce low pressure steam, the system further comprising a heat exchanger that uses a portion of the system's expended deep sea cold water to produce fresh water for irrigation or drinking. 